An inkjet printer is rapidly propagating from both personal use and business use for such reasons that full colorization can be easily achieved; that noises are low; that an image with high resolution is obtained at low costs; and that high-speed printing is possible. At present, as a recording liquid to be used for an inkjet printer, an aqueous recording liquid is the mainstream, and printed matters with high resolution become available.
Hitherto, as this aqueous recording liquid, one containing a water-soluble dye and a liquid medium as the major components was the mainstream. However, printed matters obtained by such an aqueous recording liquid were insufficient with respect to water fastness, light fastness, ozone fastness, etc. because the aqueous recording liquid contains a water-soluble dye. Then, in recent years, an aqueous recording liquid of a pigment dispersion type in which a pigment is dispersed in an aqueous medium (hereinafter sometimes referred to simply as “ink”) is developed in place of such a dye.
In recent years, following an enhancement of resolution of a printed matter, a lowering of the ink discharge amount of one time from an ink discharge nozzle is remarkable. And, because of an increase of a demand for an enhancement of a printing speed of an inkjet printer, an aqueous recording liquid of a pigment dispersion type is required to have higher pigment dispersion stability and rub fastness of printed matter. On the other hand, there is proposed a method in which a variety of water-soluble polymers or water-dispersible polymers or the like are used a pigment dispersant in the aqueous recording liquid of a pigment dispersion type.
As water-soluble or water-dispersible polymers, for example, polyvinyl alcohol, polyvinyl alcohol based block polymers containing polyvinyl alcohol as the major component, and the like are generally known. Since the polyvinyl alcohol (hereinafter sometimes referred to as “PVA”) is a crystalline polymer, if its degree of saponification is too high, its solubility in water is lowered. Furthermore, such PVA has low solubility in a general organic solvent. Accordingly, for the purpose of designing to highly functionalize PVA, there are known PVA whose degree of saponification is lowered (partially saponified PVA); PVA whose solubility in an organic solvent is enhanced by allowing a hydroxyl group of PVA to react with a compound having an aldehyde group (modified PVA); and besides, PVA based block copolymers in which a different kind of polymer is bound to PVA in a blocked state for the purpose of imparting effective characteristics as a polymer dispersant. And, it is proposed to use such a PVA based block copolymer as a polymer dispersant.
For example, as PVA based block copolymers, there are proposed ones in which a PVA based block and other block are bound to each other via a sulfur atom-containing connecting group (see, for example, Patent Documents 1 and 2); ones in which the subject blocks are bound to each other via an ether bond (see, for example, Patent Documents 3 and 4); ones in which the subject blocks are bound to each other via a nitrogen atom (see, for example, Non-Patent Documents 1 and 2); and besides, PVA based block copolymers composed of three blocks, etc. (see, for example, Patent Document 5).
[Patent Document 1] JP-A-59-189113
[Patent Document 2] JP-A-6-136036
[Patent Document 3] JP-A-2001-19770
[Patent Document 4] JP-A-2001-72728
[Non-Patent Document 1] Polymer, 39, 109 (1998)
[Non-Patent Document 2] Polymer, 39, 1369 (1998)
[Patent Document 5] JP-A-7-53841